HPHT synthesis of single crystal diamond material is well known in the art. Standard processes for manufacturing small crystals of diamond, i.e. diamond grit, involve mixing a graphite powder with a powdered metal catalyst comprising, for example, cobalt and iron (advantageously in a ratio at, or close to, the eutectic composition—65% Co:35% Fe). Other catalyst compositions are also known comprising, for example, Co, Fe, Ni, and/or Mn. A micron scale diamond powder may also be included in the reaction mixture to form seeds for diamond growth although spontaneous nucleation is possible.
In the aforementioned diamond grit synthesis process, the reaction mixture is transferred into a capsule and loaded into a press where it is subjected to a pressure of approximately 5.5 GPa and a temperature of approximately 1720 Kelvin. Such pressures and temperatures are in the region of the carbon phase diagram where diamond is the thermodynamically stable form of carbon and diamond growth occurs to form a large number of small diamond grit particles. Larger rogue crystals can form within the capsule but these are mostly highly twinned crystals with undesirable aspect ratios.
Typically, in the diamond grit synthesis process approximately constant pressure and temperature conditions are applied during diamond growth. Diamond growth to form grit particles suitable for abrasive applications may occur over a time period of a few minutes to several hours depending on the size of diamond grit particles desired for a particular application. Typical growth runs may be less than 1 hour, e.g. between 15 and 30 minutes. Typical reaction mixtures comprise approximately 50% by weight of carbon (graphite) and approximately 50% by weight of metal catalyst. As previously stated, fine diamond seeds may also be mixed with the reaction mixture to form a large number of nucleation sites within the reaction volume.
Diamond grit particles suitable for abrasive applications may range in size from, for example, 1 μm to 1 mm and the growth conditions and growth time can be controlled to produce a particular target size. Although the size of crystals coming from a single growth run will vary somewhat, the process can be controlled to obtained a reasonably uniform grit product. Subsequent processing can be utilized to separate the diamond particles according to size, weight, and/or quality.
A diamond grit process which uses small diamond seeds dispersed in a graphite powder matrix to form nucleation sites can be advantageous in producing a more controlled process yielding more consistent and uniform product when compared to a process which relies upon spontaneous nucleation within a graphite matrix. Such a seeded process works on the premise that the pressure required to grow diamond seeds is less than that required for spontaneous nucleation. Spontaneous nucleation can be undesirable as it can lead to formation of a large number of very small diamond crystals rather than larger grit particles. If a pressure P1 is the pressure required to grow on seeds and a pressure P2 is the pressure required to cause spontaneous nucleation, then it is required to operate at a pressure P3 which lies in between pressures P1 and P2. The amount that pressure P3 exceeds P1 is known as the over-pressure. This over-pressure may be controlled so as to fall within a pressure window in which diamond seed growth occurs but where widespread spontaneous nucleation is avoided, i.e. P3 is maintained between P1 and P2. As diamond growth on the seeds is driven by this over-pressure, the process is described as being pressure-driven.
During the growth process, the metal catalyst melts and the carbon dissolves in the metal catalyst and precipitates on the seeds. The metal catalyst functions as a solvent for carbon material and so is often referred to as metal solvent rather than metal catalyst. Carbon transport is via diffusion through the metal solvent. Variations in the graphite can result in nucleation sites and some spontaneous nucleation occurs away from the diamond seeds. This can be reduced by selecting good quality ordered flakes of graphite rather than disordered graphite powder.
Seeds are numerous and distributed throughout the capsule. Accordingly, carbon transport distances to individual seeds are relatively small. Regions located around the seeds become depleted in carbon as the carbon is taken out of solution during seed growth. More carbon is pulled into solution and diffuses through the depleted region. The concentration gradient in combination with the over-pressure aids in pushing the flow of carbon from the solid graphite state into solution, through the metal solvent in the depleted region, and out of solution into the solid diamond state on the seeds.
The volume of the capsule decreases during diamond growth as graphite is converted to diamond. This volume drop may be relatively large if a large quantity of carbon is converted to diamond. As reaction times are relatively short in the grit process, the rate of volume drop can be relatively high.
As diamond seeds are not anchored and are free to move around within the reaction volume, the growing seeds will tend to rise in the reaction volume under buoyancy forces from the liquid metal solvent. This can lead to inconsistent diamond crystal size and morphology. However, movement of the growing seeds under buoyancy forces can be inhibited by the presence of a graphite matrix which effectively confines the diamond particles, at least over the relatively short growth time periods required for the diamond grit process. As such, a high graphite content forming a restrictive graphite matrix coupled with well controlled and uniform pressure and temperature conditions and relatively short reaction times can give reasonably consistent crystal morphology and size for diamond grit product.
Variations of the aforementioned diamond grit process are known. For example, the small diamond seeds may be coated as described for example in WO2006/129155. Furthermore, rather than randomly distributing the small diamond seeds throughout the reaction volume the seeds may be more uniformly distributed. For example, U.S. Pat. No. 4,547,257 describes a process comprising alternating plates of graphite and metal catalyst, providing an array of holes in either the graphite or metal catalyst plates, and disposing small micron scale diamond seeds in the array of holes to form a more uniform distribution of seeds for HPHT diamond growth. EP0737510 describes the use of coated diamond seeds which may be disposed in a layered arrangement. For example, small micron scale diamond seeds may be coated with a mixture of graphite and metal catalyst, formed into compacted layers, and loaded into a HPHT capsule in a layered arrangement comprising layers of coated seeds, layers of metal solvent, and layers of graphite material. EP0528195 also discloses a HPHT capsule configuration comprising a stacked layer structure including layers of metal catalyst, layers of graphite, and layers of small micron scale diamond seed crystals. In this case, the micron scale diamond seed crystals are disposed between layers of metal catalyst. U.S. Pat. No. 6,627,168 discloses a similar stacked layer structure in which small micron scale diamond seed crystals are pressed into the surface of either a graphite layer or a metal catalyst layer. An adhesive sheet is used to transfer the seed crystals onto the graphite layer or metal catalyst layer. WO2005/084334 also discloses a stacked layer configuration in which layers of small micron scale diamond seeds are embedded in metal catalyst layers, graphite layers, or in layers comprising a mixture of metal catalyst and graphite. The seeds are transferred into the layers using one or more of the following methods: a template comprising apertures corresponding to seed positions; a transfer sheet which may be a metal catalyst layer or an adhesive layer; or a vacuum chuck. It is described that templates can be removed and reused after transfer of the seeds. If an adhesive transfer sheet is used it is described that this may be left in place within the capsule and decomposes during the initial stages of HPHT processing. Alternatively, a metal catalyst layer can be used as a transfer sheet such that the transfer sheet melts during HPHT processing.
While the aforementioned process is successful for manufacturing small diamond grit particles, the process is not suitable for manufacturing larger (>1 mm) single crystal diamonds with an acceptable morphology. Growth of larger single crystal diamonds requires fewer seeds per mass of carbon source material such that a larger quantity of carbon is available for transport to each seed. Furthermore, longer reaction times are required to grow larger crystals and carbon transport distances are increased. If the grit process is run with fewer seeds for longer time periods, as the graphite becomes depleted the growing diamond seeds become more mobile within the reaction volume, being less restricted by the graphite matrix, and the seeds move upwards under buoyancy forces within the liquid metal solvent. As the orientation of the seeds varies which respect to the applied pressure, and/or the distance between the seeds and the graphite material is variable and ill-controlled, the seeds tend to grow with ill-defined morphologies if the process is run for the longer time periods required to fabricate large single crystal diamonds. Furthermore, it has been found to be difficult to control the applied pressure over long periods of time using this process such that an over-pressure is maintained for seed growth without exceeding the pressure limit at which widespread spontaneous nucleation occurs. That is, the previously described pressure window for this process between P1 (the pressure at which diamond seed growth occurs) and P2 (the pressure at which widespread spontaneous nucleation occurs) is relatively narrow and it is difficult to maintain an operating pressure P3 so as to be maintained within this operating pressure window over the long periods of time required for large single crystal diamond growth.
In light of the above, an alternative method is utilized in the art for growth of larger single crystal diamonds. The standard method for manufacturing larger single crystal HPHT diamond material is known in the art as the temperature gradient method. This method is similar to the previously described diamond grit process in that the reaction mixture comprises a graphite powder (graphite flakes or a diamond grit could alternatively be used) and a metal catalyst. However, instead of using a micron scale diamond powder to seed the reaction mixture, a seed pad is manufactured comprising a one or more single crystal diamond seeds anchored to, or embedded in, an inert holder which may be formed by a ceramic disk. The seeds themselves are larger in size than the micron size diamond powder used to seed grit processes, typically 0.5 mm or greater, and are selected to have a desired morphology and orientation. The seed pad, which is prepared from a chemically inert ceramic material such as MgO, is introduced into a capsule and the reaction mixture is disposed over the seed pad within the capsule. The capsule is then loaded into a press and subjected to a HPHT treatment.
The temperature gradient method is further distinguished over the diamond grit process in that while a relatively constant pressure is maintained over at least a majority of a growth run, the capsule is heated to a higher temperature at the top of the capsule than at the bottom of the capsule. Thus a temperature gradient is formed across the capsule from top to bottom and it is this temperature gradient which drives carbon transport and diamond seed growth. Hence this process being known as the temperature gradient method.
The temperature gradient method differs further from the previously described pressure driven grit process in the chemistry of the reaction mixture. Typically, much less carbon is provided in the reaction mixture which may comprise approximately 10% by weight of carbon (graphite) and approximately 90% by weight of metal solvent. Furthermore, the metal solvent may differ according to certain processes although similar compositions to those used for the grit process may be utilized including, for example, cobalt-iron eutectic compositions or other compositions comprising, for example, Co, Fe, Ni, and/or Mn.
The capsule configuration for the temperature gradient method also differs from that used in the grit process in that a single seed pad is located in a lower region of the capsule in a horizontal orientation. The reaction mixture is located over this seed pad. In practice, one or more layers of metal catalyst strips may be provided over the seeds forming a layer a few millimeters thick with the remaining reactants disposed thereover as a mixture. The carbon composition of the metal strips is reduced when compared to the carbon content in the overlying mixture, e.g. a precisely controlled carbon content of a few percent by weight. The reason for this arrangement is to reduce the carbon concentration in contact with the seeds at the start of the run as this prevents adverse effects taking place when the carbon is transformed to diamond. The capsule design is such as to give uniform radial temperature distribution. This is achieved through design of heating elements and insulating materials.
The temperature gradient method may be defined as comprising two main stages. In a first stage graphite is converted to fine diamond crystals by application of pressure and temperature to dissolve graphite in the metal solvent and crystallize diamond by spontaneous nucleation. As an alternative, fine diamond crystals may be provided as a source of carbon from the outset.
The fine diamond crystals are buoyant in the metal solvent and rise to an upper region of the capsule thus forming a three layer system: a top layer of fine diamond crystals; an intermediate layer primarily comprising carbon saturated metal solvent; and a lower portion comprising the seed pad.
In a second stage of the temperature gradient process diamond seed growth occurs. A high temperature in the upper region of the capsule causes diamond crystals to dissolve. The equilibrium concentration of carbon is higher at the hotter end of the capsule than at the cooler end. Dissolved carbon diffuses downwards and a lower temperature at the seed pad causes carbon to come out of solution at the seeds resulting in diamond growth on the seeds. While there is a relatively large volume drop during the first stage as graphite material is converted to diamond material via spontaneous nucleation to form a diamond material as the carbon source for seed growth, the reaction volume remains fairly stable during the second stage of diamond seed growth as the reaction involves diamond-to-diamond conversion.
While not being bound by theory, it is believed that although carbon transport may be partially driven by a carbon concentration gradient between upper and lower regions of the capsule, this mechanism cannot wholly account for the levels of carbon transport observed in the temperature gradient method. Secondary ion mass spectrometry (SIMS) analysis indicates that the concentration gradient of carbon is very small along most of the capsule. Accordingly, it would appear that Fick's diffusion alone (dC/dx) cannot explain the rate of carbon transport. As such, it is believed that the temperature dependent Soret diffusion term (dT/dx) is dominant over the length of the capsule. Soret diffusion (dT/dx) thus drives the process such that the rate of carbon transport to the seeds is increased as the temperature gradient is increased. Time modelling of this process indicates that observed rates of carbon transport over the length of the capsule can only be accounted for using this mechanism. In contrast, in the local vicinity of a seed, a region of carbon depleted material forms, sometimes known as a “carbon depletion zone” or “carbon denuded zone”, which can to some extent limit seed growth rate. It is believed that the larger carbon concentration gradient in the immediate vicinity of the seed crystals is dominated by Fick's diffusion although this diffusion constant cannot sustain carbon transport by concentration gradient alone.
Seed growth is thus driven by the temperature differential and the length scale driving dissolution of carbon (diamond) at the top region of the capsule and precipitation of carbon onto the seed in the lower region of the capsule. Furthermore, it is believed that carbon transport is largely diffusion based rather than convection based although some temperature driven convention currents may occur (although these will be limited because the hotter material is at the top of the capsule from the outset). It is also worth noting that diamond growth doesn't occur at the seed crystals if the temperature gradient is reversed, i.e. hotter at the bottom of the capsule, and that the temperature gradient is always aligned with the direction of gravity. This is important as undesirable spontaneously nucleated diamond that forms in the catalyst between the diamond source and the seed crystals will tend to migrate through buoyancy back to the top of the capsule (i.e. where the carbon source material is located). Furthermore, attempts to grow in a radial direction have been largely unsuccessful for similar reasons.
An important feature of the temperature gradient process is that the seeds are anchored to a pad in a lower portion of the HPHT capsule to ensure that the seeds have a fixed and well defined orientation relative to the applied temperature and pressure. That is, the growing diamond crystals are prevented from floating within the metal solvent during synthesis and this allows the crystals to grow with a well defined single crystal morphology. If the seeds are allowed to float in the melted reactants during synthesis, this leads to misshapen growth. Furthermore, buoyancy would otherwise drive the seeds to the top end of the capsule i.e. to where the carbon source material is located. Therefore anchoring is required to form good morphology, large single crystal diamond material. The temperature gradient allows carbon to be transported to the anchored crystals to achieve large single crystal diamond growth. Diamond growth is driven by the temperature differential. A larger temperature gradient will, to first order, increase the growth rate of diamond.
Another important feature of the temperature gradient process is that all the seeds must be placed at the same level in the temperature gradient so as to be exposed to the same growth conditions and thus obtain uniform product. That is, a single seed pad is provided and located at a position within the temperature gradient such that all the seeds on the pad are exposed to substantially the same temperature. Additionally, seed spacing is important as non-uniformly spaced seeds can also result in non-uniform growth rates.
FIG. 1 illustrates a capsule arrangement in a HPHT press for a temperature gradient process. The HPHT press comprises anvils 2. A capsule 4 is loaded within the HPHT press. The capsule 4 includes a seed pad 6 on which diamond seeds 8 are disposed. Reactants 10 including a carbon source material and a metal catalyst are disposed over the seed pad. A temperature difference between the top and bottom sides of the capsule (T2>T1) is generated and maintained to drive growth. The temperature gradient method is capable of forming a plurality of relatively large single crystal diamonds in a single process run. However, the number of single crystal diamonds is limited to the number which can be mounted to the seed pad and/or the size of crystal that is ultimately required. The temperature gradient may be matched to the seed size and distribution. In this regard, it may be noted that there is an inter-play between the number of seeds, the magnitude of the temperature gradient, and the tendency to form inclusions. For example, if metal inclusions within the diamond material grown on the seeds are to be avoided it is known that the temperature gradient must be reduced as the number of seeds per unit area on the seed pad is reduced.
It should be appreciated that both the diamond grit process and the temperature gradient process have been the subject of many years of research by numerous groups and that both processes have been carefully optimized for their respective purposes, i.e. large quantities of diamond grit material for abrasive applications and lower quantities of large synthetic single crystal diamonds for a range of applications including optical, thermal and mechanical applications. As such, the aforementioned description of these processes is only intended to provide an over-view in order to set the context for the present invention.
Modifications to the temperature gradient method have been proposed for increasing the number of large single crystal diamonds which can be formed per HPHT process run. For example, a multi-layer temperature gradient method may be envisaged by stacking a plurality of seed pads into a single HPHT capsule with carbon/metal solvent powder disposed between each of the layers. However, this approach is considered to be problematic as the absolute temperature at the seed pads will be different for each layer therefore resulting in different growth morphologies. Since the temperature window for optimum growth is small, this is likely to result in poor growth or perhaps no growth at all. The metal solvent composition could potentially be varied such that the eutectic temperature is adjusted to compensate this problem. However, such arrangements are not considered particularly successful.
Another alternative way which may be envisaged to solve the problem of providing multiple seeds pads in a temperature gradient method is to provide a more complex heating arrangement in which separate heating elements are applied to the layered structure in order to try and provide uniform growth conditions at each of the seeds pads and effectively provide a plurality of zones, each having their own temperature gradient. However, it is difficult to vary the temperature in this fashion in any practical arrangement due partly to the relatively high aspect ratio of the seed pads and solvent catalyst. Accordingly, while this is conceptually possible, in practice it is difficult to configure and control such a system to ensure that each seed grows in a uniform manner.
In contrast to the temperature gradient seed-pad processes described above, a pressure driven seed-pad configuration has previously been proposed in the art by Masao Wakatsuki and co-workers at the Institute of Materials Science, University of Tsukuba who have published several academic papers and patent applications in this area including: (1) Masao Wakatsuki “Formation and Growth of Diamond—For Understanding and Better Control of The Process” Rev. High Pressure Sci. Technol., Vol. 7 (1998) 951-956; (2) JP 63-084627; (3) Masao Wakatsuki and Kaoru Takano “Suppression of spontaneous nucleation and seeded growth of diamond”, High-Pressure Research in Mineral Physics, pp 203-207 (1987); (4) Y. Wang, R. Takanabe and M. Wakatsuki, “The stability of the regrowth-treated carbon source in the excess pressure method of growing diamonds”, High Pressure Science and Technology, Proceedings of the Joint 15th AIRAPT and 33rd EHPRG International Conference, Warsaw, Poland, Sep. 11-15, 1996, ed. By W. A. Trzeciakowski, World Scientific Publ. Co., London, 1996 pp. 565-567; (5) JP59-203717; (6) JP54-069590; and (7) Y. Wang et al. “Crystal growth of diamond from regrowth-treated graphite”, Advances in New Diamond Science and Technology, 521-524, MY, Tokyo, 1994.
These prior art documents published in the 1980's and 1990's identify the problem that it is difficult to control a pressure driven process to grow single crystal diamond material on seeds while at the same time avoiding spontaneous nucleation of diamond growth in the graphite matrix. As previously described in relation to the seeded diamond grit process, the pressure P1 required to grow diamond seeds is less than the pressure P2 required for spontaneous nucleation. As such, if controlled seed growth is to be achieved it is required to operate at a pressure P3 which lies in between pressures P1 and P2. However, it is difficult to control the applied pressure over long periods of time using this process such that an over-pressure is maintained for seed growth without exceeding the pressure limit at which widespread spontaneous nucleation occurs. That is, the previously described pressure window between P1 and P2 is relatively narrow and it is difficult to maintain an operating pressure P3 so as to be maintained within this operating pressure window over the long periods of time required for large single crystal diamond growth.
Masao Wakatsuki and co-workers propose a solution to this problem which utilizes a two step process comprising: (i) surface regrowth of graphite at a pressure below that required for diamond growth; and (ii) subsequently increasing the pressure to achieve diamond seed growth at a raised pressure. It is described that in the first step of the method source graphite stays largely unchanged except for being covered with regrown graphite particles over its surface. It is described that the regrown graphite material functions to absorb dissolved graphite, decreasing supersaturation for nucleation or growth of diamond through a kinetic balance between absorption and supply from the raw graphite. It is stated that this mechanism results in a buffer effect on the supersaturation for nucleation or growth of diamond against a change of reaction pressure and thus the rate of nucleation and growth is easily kept stable by the presence of regrown graphite particles, even if the reaction pressure is varied a little.
Masao Wakatsuki and co-workers thus suggest that such a two step process can be used to increase the size of the pressure window between P1 and P2 allowing an operating pressure P3 to be maintained within this pressure window during the second step to achieve controlled growth of diamond seeds in a pressure driven process. Furthermore, they demonstrated such growth in HPHT capsule configurations including two seeds, one located in a lower region of the HPHT capsule and one located in an upper region of the HPHT capsule. In certain configurations the seeds are disposed between graphite and metal catalyst (flux) layers and are not anchored to a seed pad. In certain other configurations the seeds are embedded in respective seed pads, i.e. an upper and lower seed pad are provided with a seed anchored to each pad.
While such a process and HPHT capsule construction would appear to open the possibility of running a pressure driven growth process over long periods of time to achieve growth of large single crystal diamonds, Masao Wakatsuki and co-workers found that this was not possible and identified a major problem with their approach. In particular, Masao Wakatsuki and co-workers found that while their method was successful at reducing spontaneous nucleation and achieving controlled diamond seed growth, the seed growth terminates after a certain length of time and they found it impossible to grow over long periods of time to achieve large single crystal diamond, e.g. greater than 2 mm. They attributed this termination mechanism to the regrown graphite. It is taught that the regrown graphite coating the original graphite source material does not act as a carbon supply itself for diamond growth and continues growing during diamond growth eventually forming a dense layer over the source graphite and terminating diamond growth by cutting off the carbon source.
As such, Masao Wakatsuki and co-workers present a conundrum. They teach that regrown graphite can be provided to alleviate the problems of spontaneous nucleation in a pressure driven diamond growth process. This is required to achieve controlled seed growth of large single crystal diamonds having uniform size and morphology. However, they teach that regrown graphite functions to terminate diamond seed growth prior to achieving large single crystal diamonds. It is perhaps for this reason that the temperature gradient method has remained the standard process for growing large synthetic HPHT single crystal diamond material.
In light of the above, it is an aim of certain embodiments of the present invention to provide an alternative approach to increasing the number of relatively large single crystal diamonds which can be grown in a single HPHT synthesis run. In particular, it is an aim of certain embodiments of the present invention to achieve this goal while also retaining a level of uniformity in diamond growth and a relative simplicity in process configuration and control which is difficult or impossible to attain using the previously described approaches. Accordingly, certain embodiments aim to achieve the following targets: (i) synthesis of single crystal diamonds which are larger than those achievable using the basic HPHT diamond grit configuration and larger than those achievable using the two-step process described by Masao Wakatsuki and co-workers; (ii) synthesis of a larger number of single crystal diamonds per growth run than is achievable using the standard temperature gradient method; and (iii) synthesis of large single crystal diamonds which have a relatively uniform size and morphology using a manufacturing configuration which is more simple to operate and control in a reproducible and uniform fashion when compared to the previously described methods.